1. Field of the Invention
The present invention relates to a process for producing a hydrated iron oxide powder used for producing an iron type acicular magnetic powder which is widely used as a magnetic powder for a magnetic recording medium.
2. Description of the Prior Art
It is well known that important characteristics of a magnetic powder for magnetic recording include a coercive force of a magnetic powder, a dispersibility of a magnetic powder in a magnetic composition for coating and an orientation of a magnetic powder in a magnetic recording tape. As it is well known, these characteristics are important factors which highly affect to the characteristics of the magnetic recording tape. The configuration of the magnetic powder directly relates to these characteristics of the magnetic powder. For example, when the coercive force is given by the shape anisotropy as the conventional magnetic recording medium, the coercive force is increased depending upon the improvement of the acicular property of the particles. The dispersibility is improved by decreasing branched or curved configuration whereby the steric hindrance is decreased to improve the orientation and accordingly, the recording characteristics of the magnetic recording tape are improved. It is important to obtain a magnetic powder having excellent acicular property with smaller branched configuration in the preparation of the magnetic powder for a magnetic recording medium. In the industrial process, hydrated ferric oxide, mainly an acicular particle of goethite .alpha.-FeOOH is firstly produced as a starting material and is converted into a magnetic metallic iron powder by heating, dehydrating and reducing it or is converted into Fe.sub.3 O.sub.4 powder by partially reducing it or is converted into .gamma.-Fe.sub.2 O.sub.3 powder by oxidizing Fe.sub.3 O.sub.4 powder. These processes have been usually employed.
The magnetic powders obtained by these processes have configurations depending upon the configuration of the starting material of the acicular goethite. Therefore, in order to obtain a magnetic powder having a desired configuration, it is necessary to control the configuration of the goethite powder. However, in the process of the reduction of the powder, dehydration and the migration of atoms occurs, by which powder are deformed or sintered. Therefore, it is necessary to give a powder which is not easily deformed or sintered each other. It has been found that the deformation of particles and the sintering of particles are remarkably caused if the growth rate of the crystalites of the magnetic particles is remarkable in the step of the reduction.
On the other hand, it is found that the growth of crystalites of the magnetic particles is remarkably inhibited by Si component. There are many proposals based on the phenomenon as disclosed in Japanese Examined Patent Publication No. 19541/1977; and Japanese Unexamined Patent Publication No. 95097/1977; No. 134858/1977; No. 4895/1978; No. 30758/1977; No. 77900/1976; No. 121799/1977; and No. 153198/1977. Various processes for incorporating Si component in the acicular non-magnetic powder as the starting material can be considered as proposed. In the fundamental consideration, these processes can be classified into a process for growing crystals in the presence of Si component in the step of the production of the goethite as the starting material (referring to as A process) and a process for adsorbing Si component as a post treatment, after producing a non-magnetic powder (referring to as B process).
The A process is superior to the B process in view of the uniform incorporation of the Si component, however, it has disadvantage that the growth of the geothite crystals is inhibited to be difficult to obtain the particles having the desired size if the Si content is increased. The B process has not any difficulty on the control of the particle size, however, it is difficult to adsorb uniformly the Si component and the bonding of the Si component to the surface of the particles is not so strong to effectively impart the effect of the Si component. On the contrary, the present invention is to combine both the advantages to overcome the disadvantages so as to attain the superior Si treatment for the goethite.